Thanks for the quick reply.  Having crawled around in the sugar scoop space aft, there does appear to be a breather or relief pipe but ….. it terminates just inside the filler cap so once the cap is on, no air would get in.  I’ve inspected it but only to ascertain if it was some kind of jury rig and it seems to be as built. As far as the pump is concerned, the noise itself is not so bothersome, its the fact that the pump operates every 2-3 mins even without demand so for me to get any sleep, I have to switch it off.